Dzhokhar Tsarnaev, who is charged in connection with the Boston Marathon bombings, is shown in a photo from the Russian social networking site Vkontakte. / vk.com via AP

by Kevin Johnson, Donna Leinwand Leger and Aamer Madhani, USA TODAY

by Kevin Johnson, Donna Leinwand Leger and Aamer Madhani, USA TODAY

BOSTON -- The surviving suspect in last week's Boston Marathon bombings began responding to investigators' questions Sunday evening, marking a dramatic turn for law enforcement officials trying to piece together why two brothers born near war-torn Chechnya allegedly carried out an attack on their adopted country.

Investigators had been unable to question Dzhokhar Tsarnaev, who was badly wounded and unable to talk since he was captured Friday night. But less than 48 hours after he was taken into custody, the 19-year-old suspect - who remains hospitalized in serious condition - began responding to questions in writing, according to a law enforcement official who was not authorized to discuss the matter and spoke on the condition of anonymity.

The official declined to offer any details about the exchanges but said Tsarnaev was providing "substantive" information, even as investigators prepare to levy charges against him as soon as today. Authorities also said that the suspect's neck wound may have been self-inflicted and an attempt at suicide sometime prior to his capture.

The latest turn in this case comes on a day when U.S. lawmakers raised questions about whether authorities missed warning signs about the immigrant brothers - and as Boston regained a semblance of normalcy nearly a week after the horrific attack and the ensuing manhunt that locked down the city.

Reps. Michael McCaul, R-Texas, chairman of the House Homeland Security Committee, and Pete King, R-N.Y., said Sunday that they want federal officials to explain why the elder brother was not pursued further after he was questioned by authorities in 2011. The Russian government had asked for an investigation of Tamerlan Tsarnaev, who died in a shootout Friday, out of concern that he had ties to militant separatist groups in southern Russia.

McCaul and King noted in a letter to the heads of the FBI, Department of Homeland Security and the Office of the Director National Intelligence that the Boston bombings marks the fifth time in recent years that someone under FBI investigation has gone on to be involved in a terrorist attack. Other suspects that the FBI have questioned - but not detained - that have gone on to take part in violence include U.S.-born cleric Anwar al-Awlaki, who would become al-Qaeda in the Arabian Peninsula's chief propagandist, and Nidal Hasan, the U.S. Army major charged with killing 13 people in 2009 at Fort Hood in Texas.

"They raise the most serious questions about the efficacy of federal counterterrorism efforts," McCaul and King wrote.

Rep. Mike Rogers, R-Mich., chairman of the House Intelligence Committee and a former FBI agent, defended the bureau's work.

"They had information from a foreign intelligence service that they were concerned about his possible radicalization," he said on the NBC's Meet the Press. "The FBI did their due diligence and did a very thorough job of trying to run that down, and then asked for some more help from that intelligence service to try to get further clarification, and unfortunately that intelligence service stopped cooperating." Rogers did not identify the intelligence agency.

Still, the Boston bombing that killed three and injured more than 170 others may represent the type of blind spot that federal law enforcement officials have long feared.

In more than a decade since the Sept. 11 attacks, FBI Director Robert Mueller has repeatedly expressed the concern that has most worried counterterrorism authorities tasked to prevent another U.S. assault. "The problem that we have is not what we know, it's what we don't know and I do fear another attack," Mueller said in a 2006 CNN interview.

That fear, which Mueller has related countless times to congressional committees and his own top lieutenants was grounded in a chilling reality: that despite all the money and manpower poured into a post-9/11 effort to make the U.S. more secure, there is no way to foresee and block every plot, especially one that may involve determined suspects who emerge from off the national grid.

UNDER THE RADAR

Though two years ago the FBI interviewed one of the suspects, Tamerlan Tsarnaev, nothing surfaced in a two-month review that was requested by a Russian intelligence service. Russian authorities were concerned about Tsarnaev's possible links to radical Islam and Chechen extremists, prior to the suspect's extended trip to the country last year.

"There just wasn't anything there," said a federal law enforcement official who has been briefed on the matter. "We ask that the government get back with us if they develop new information, but they did not. The Russians seemed satisfied, so we closed it."

At no time prior to the bombing were Tamerlan Tsarnaev and his brother, Dzhokhar - among the thousands contained on government watch lists - seen as potential terror suspects. Neither was there any advance information that Boston's iconic footrace might be a target for an assault, authorities say.

Philip Mudd, a former FBI and CIA counterterrorism official, said the Boston attacks more closely resemble "Columbine than any connection to al-Qaeda."

Referring to the 1999 Colorado high school shooting that involved two young attackers, Mudd said the Boston case similarly involves attackers armed with commonly available weapons and explosives who worked in a "closed cluster" that is most difficult for law enforcement to penetrate in advance.

Though authorities continue to search for possible broader international connections, Mudd said key aspects of the operation - the acquisition of widely available materials, the absence of an apparent escape plan, little attempt to conceal themselves and the apparent lack of major funding - all point to a more self-contained plot.

"These guys didn't even bother to obscure their faces," Mudd said.

A RADICAL TURN?

Authorities are also trying to figure how and when the suspects may have been radicalized. While family members have said Tamerlan Tsarnaev had trouble adjusting to life in the USA, his brother, by all accounts, was popular and acclimated in Boston.

The suspect's uncle, Ruslan Tsarni, recalled in an interview with WUSA-TV in Washington that he had a falling out with Tamerlan Tsarnaev in 2009 after the young man spoke about "not ... having a sense in life" and that he believed his actions should be dictated by "God's will."

The two brother's activity on the Internet also suggest they had developed an interest in radical Islamist figures some time ago.

In August 2012, soon after authorities believe he returned from a six-month visit to Russia, the elder Tsarnaev created a YouTube channel with links to a number of videos with ties to radical Islamist causes. One features the firebrand Australian cleric Feiz Mohammed.

A militant group in war-torn Chechnya - Mujahideen of the Caucasus Emirate Province of Dagestan - took the step of issuing a statement on Sunday to disassociate itself with the Boston bombing suspects.

Dzhokhar Tsarnaev was active on Twitter, where most of his postings appeared to be innocent messages to friends. But the SITE Intelligence group, which monitors Islamist websites, notes that he followed someone with the account "Al_firdausiA," which translates to "the highest level of Paradise, Allah willing." Among that user's tweets was a message encouraging readers to listen to an audio series by the late radical terrorist Awlaki.

THE WAY FORWARD

Many questions remain on how authorities will proceed in handling the living suspect. Massachusetts does not have a death penalty, but there is a groundswell of support among congressional lawmakers to lay terrorism charges in federal court, which would carry the death penalty. The FBI has been leading the investigation since the bombing and has been directed by President Obama to treat it as an "act of terrorism."

The Obama administration has raised eyebrows among some civil libertarians by deciding not to read Dzhokhar Tsarnaev his Miranda rights, which notify a suspect of the right to remain silent and to have legal counsel. Authorities say Miranda is being withheld from Tsarnaev under the public safety exception, which U.S. Attorney for Massachusetts Carmen Ortiz has said officials are invoking because of an immediate threat to the public.

But that exception is not meant to be open-ended, says Anthony Romero, executive director of the American Civil Liberties Union.

"We must not waver from our tried-and-true justice system, even in the most difficult of times. Denial of rights is un-American and will only make it harder to obtain fair convictions," Romero said.

Sen. Lindsey Graham, R-S.C., argued on CNN Sunday that Tsarnaev, who was born in Kyrgyzstan and is a naturalized U.S. citizen, should be held as an enemy combatant for interrogation purposes. That would allow authorities to take their time gleaning information from him. But Graham says that Tsarnaev is not eligible to be tried by a military commission because he was not caught on a foreign battlefield.

"Most Americans want to find out what he knew, who he associated with, does he know about terrorist organizations within or without the country that are trying to hurt us?" Graham said. "Does he know about a future attack?"